The present invention relates to a ball screw mechanism in which, even when a retaining piece is interposed between two mutually adjoining load balls, decrease in the number of load balls can be controlled as much as possible to thereby prevent the load balls from degrading the load capacity and rigidity thereof, can reduce friction between the load balls and retaining piece to thereby enhance the circulating performance of the retaining piece, and can prevent the load balls from butting against each other to thereby prevent degradation in the operation efficiency thereof, deterioration in the quality of sounds generated, and the friction and damage of the load balls.
Conventionally, in a ball screw mechanism, as shown in FIG. 9, on the outer peripheral surface of a screw shaft 1 and on the inner peripheral surface of a nut 2, there are respectively formed spiral-shaped screw grooves 3 and 4 which disposed opposed to each other and, in a screw groove circulation passage defined by the two screw grooves 3 and 4, there are disposed a large number of load balls 5 in such a manner that they are free to roll. In case where the screw shaft 1 and nut 2 are rotated with respect to each other to thereby move one of them in the axial direction thereof, the screw shaft 1 and nut 2 are smoothly moved in a spiral manner with respect to each other.
In such ball screw mechanism, the load balls 5 are arranged densely within the screw grooves 3 and 4 and they roll in the same direction within the individual screw grooves 3 and 4; and, in the rolling movements thereof, at contact points between the two mutually adjoining load balls 5 and 5, the load balls 5 rolling in the mutually opposite directions are contacted with each other to thereby interfere with their mutual rolling movements, with the result that the load balls 5 and 5 are caused to slip at the contact points. This interferes with the free rolling movements of the load balls 5, degrades the operation efficiency of the load balls 5, gives rise to friction and damage in the load balls 5, causes variations in the torque of the load balls 5, and swells the noises that are produced by the load balls 5.
To cope with these problems, as shown in FIG. 10, between the mutually adjoining load balls 5, there are respectively interposed spacer balls 6 each having a diameter smaller by several tens xcexc than the load balls 5. That is, such interposition of the spacer balls 6 prevents the load balls 5 against slippage, improves the operation efficiency of the load balls 5, and reduces the friction and damage of the load balls 5 to thereby prevent variations in the torque.
However, in the ball screw mechanism shown in FIG. 10, while the load balls 5 are, for example, ten in number, the spacer balls 6 are also, for example, ten in number. Therefore, when compared with the ball screw mechanism shown in FIG. 9, a clearance between the two adjoining load balls 5 is large and the load balls 5 are reduced in number down to about one half, which decreases the load capacity of the ball screw mechanism and also degrades the rigidity thereof.
In view of the above problems, there is also known a structure in which, as shown in FIG. 11, between every two mutually adjoining load balls 5, there is interposed a retaining piece 10 having two concave surfaces 11 respectively facing the two load balls 5. According to this structure, while the load balls 5 are in contact with the concave surfaces 11 of the retaining piece 10, they are allowed to circulate well within the spiral-shaped screw grooves 3 and 4. Therefore, when compared with the conventional structure using spacer balls, the spacer, that is, the retaining piece 10 can be made thinner, which makes it possible to control reduction in the number of load balls and thus avoid degradation in the load capacity and rigidity of the ball screw mechanism.
However, in the ball screw mechanism shown in FIG. 11, since the relation between the outside dimension of the retaining piece 10 and the outside dimension of the load ball 5 is not always considered properly, there is a fear that, when the retaining piece 10 passes through the spiral-shaped screw groove circulation passage and tube circulation passage, it can interfere with the inner walls of these circulation passages. Therefore, such interference makes it difficult for the load balls 5 to circulate stably, thereby giving rise to generation of variations in the torque, or causing the retaining piece 10 to wear.
The present invention aims at eliminating the drawbacks found in the above-mentioned conventional ball screw mechanisms. Accordingly, it is an object of the invention to provide a ball screw mechanism which prevents the retaining piece from interfering with the inner walls of the circulation passages to thereby allow the load balls to circulate stably, thereby preventing not only variations in the torque but also the wear of the retaining piece for enhancement of the durability of the retaining piece.
In attaining the above object, according to a first aspect of the invention, there is provided a ball screw mechanism structured such that, on the outer peripheral surface of a screw shaft and on the inner peripheral surface of a nut, there are respectively formed mutually facing spiral-shaped screw grooves, there are rollably disposed a large number of load balls in a spiral-shaped screw groove circulation passage formed by these two screw grooves, and, to the screw groove circulation passage, there is continuously connected a return circulation passage through which load balls taken out from the screw groove circulation passage can be returned again into the screw groove circulation passage, wherein, between every two mutually adjoining ones of the large number of load balls, there is interposed a retaining piece having two concave surfaces respectively facing the two mutually adjoining load balls, the retaining piece has an outside diameter dimension equal to or more than 0.5 times the outside diameter dimension of the load ball, and the retaining piece has an outside diameter dimension which, when the retaining piece passes through the screw groove circulation passage and tube circulation passage, prevents the retaining piece from touching the inner walls of the screw groove circulation passage and tube circulation passage.
Also, according to a second aspect of the invention, there is provided a ball screw mechanism structured such that, on the outer peripheral surface of a screw shaft and on the inner peripheral surface of a nut, there are respectively formed mutually facing spiral-shaped screw grooves, there are rollably disposed a large number of load balls in a spiral-shaped screw groove circulation passage formed by these two screw grooves, and, to the screw groove circulation passage, there is continuously connected a return circulation passage through which load balls taken out from the screw groove circulation passage can be returned again into the screw groove circulation passage, wherein, between every two mutually adjoining ones of the large number of load balls, there is interposed a retaining piece having two concave surfaces respectively facing the two mutually adjoining load balls, and the retaining piece has an outside diameter dimension in the range of 0.5 to 0.9 times the outside diameter dimension of the load ball.
As described above, according to the invention, as a first condition, the retaining piece has an outside diameter dimension equal to or more than 0.5 times the outside diameter dimension of the load ball. This is basically because, in case where the retaining piece has an outside diameter dimension less than half the outside diameter dimension of the load ball, the retaining piece cannot be lifted up from between the two mutually adjoining load balls, which disables the retaining piece to fulfil the expected function thereof.
Also, as a second condition, the retaining piece has an outside diameter dimension which, when the retaining piece passes through the screw groove circulation passage and tube circulation passage, prevents the retaining piece from touching the inner walls of these screw groove circulation passage and tube circulation passage, or has an outside diameter dimension which is equal to or less than 0.9 times the outside diameter dimension of the load ball. This is basically because, to circulate the load balls stably, the concave surface holding allowance for the load balls (the steel ball holding allowance) maybe preferably set large, which increases the outside diameter of the retaining piece accordingly; but, in case where the concave surface holding allowance is excessively large, the retaining piece interferes with the inner walls of the screw groove circulation passage and return circulation passage having various curvatures, thereby degrading the operation efficiency of the retaining piece. Under these circumstances, to balance them with each other, the outside diameter of the retaining piece is set as large as possible on one side but, on the other hand, the outside diameter of the retaining piece is controlled to such a degree that it does not interfere with the inner walls of the circulation passages.
As described above, according to the invention, since, between every two mutually adjoining load balls, there is interposed a retaining piece having two concave surfaces respectively facing the two load balls, even in case where a retaining piece between two mutually adjoining load balls, the number of load balls can be reduced as many as possible without degrading the load capacity and rigidity of the ball screw mechanism; and, friction between the load balls and retaining pieces can be reduced as much as possible, so that not only the circulating performance of the retaining piece can be enhanced, but also the load balls can be prevented from butting each other, thereby preventing the degraded operation efficiency of the load balls, generation of noises by the load balls, the deteriorated quality of sounds generated by the load balls, and the friction and damage of the load balls.
In addition, because the relation between the outside diameter dimension of the retaining piece and the outside diameter dimension of the load balls is properly considered, the interference of the retaining pieces with the inner walls of the circulation passages can be prevented, thereby allowing the load balls to circulate stably. This can prevent variations in torque as well as can prevent the wear of the retaining pieces to thereby enhance the durability thereof.